Fuel cells are electrochemical cells that are being developed for mobile and stationary electric power generation. One fuel cell design uses a solid polymer electrolyte (SPE) membrane or proton exchange membrane (PEM), to provide ion transport between the anode and cathode. Gaseous and liquid fuels capable of providing protons are used. Examples include hydrogen and methanol, with hydrogen being favored. Hydrogen is supplied to the fuel cell anode. Oxygen (as air) is the cell oxidant and is supplied to the cell's cathode.
The membrane-electrode assembly (MEA) in a polymer electrolyte/proton exchange membrane fuel cell comprises the electrodes (anode and cathode), with a thin layer of catalyst, bonded to either side of the membrane. In principle, these materials act together to conduct electrons (from the anode to an external resistive circuit) from H2 oxidation, transport H+ (through the membrane), and conduct electrons to the cathode to the oxygen reduction catalyst to recombine H+ with O2. In practice, however, these materials are not optimally tuned for peak performance.
Nafion® is a widely used proton-conducting material because it possesses high proton conductivity, good mechanical strength, and good chemical and electrochemical stability under fuel cell operating conditions. These properties arise from the chemical compositions of Nafion® membranes which are a family of perfluorosulfonic acid ionomer membranes. Related ionomer materials include Flemion® and Aciplex® (produced by Asahi Glass and Asahi Kasei, respectively) and a Dow® ionomer. The backbones of the polymers comprise tetrafluoroethylene monomer units and trifluoroethylene monomer units that provide excellent oxidative stability to the membrane. Side chains of two or more —OCF2CF2— moieties (or analogs of these moieties) are attached to the backbone and each of these side chains is terminated by a sulfonic acid ion (SO3−). The total number and proportion of tetrafluoroethylene moieties and trifluoroethylene moieties per polymer molecule provides the membrane with its properties including a suitable abundance of pendant ionizable groups (e.g. sulfonate groups) for transport of protons through the membrane from the anode to the cathode when the membrane is suitably hydrated. The large electronegativity of the fluorine atoms bonded to the same carbon atom as the sulfonic acid group makes this group strongly acidic. In the example of Nafion 117 membrane, the equivalent weight (g ionomer/mol SO3−1) is about 1100.
The fuel cell electrodes are generally composed of a mixture of high surface area carbon-supports (0.1 to 1 micrometer particles or particle agglomerates) on which nanometer sized electrochemically active catalyst particles are supported (typically 2 to 20 nanometer sized platinum, platinum-group metals, or alloys thereof, as well as alloys of platinum or platinum-group metals with transition metals) and binder. Binders are typically perfluorinated polymers (e.g., PTFE, FEP, etc.) or sulfonic acid based polymers (e.g., perfluorosulfonic acid ionomers). These electrodes are sandwiched between the ionic membrane and porous conductive materials, such as woven graphite, graphitized sheets, or carbon paper (commonly referred to as diffusion media or gas diffusion layers). Depending on whether electrodes are supported on the ionomeric membrane or on the gas diffusion layer, the assemblies are referred to as catalyst-coated membranes or catalyst-coated diffusion media (also called gas diffusion electrodes), respectively. The electrochemical fuel cell reactions—the anodic oxidation of hydrogen or other fuels and the cathodic reduction of oxygen (from air)—occur throughout the electrodes on the active catalyst particles. For the electrochemical reactions to occur, protons need to be conducted throughout the electrodes into and out of the ionomeric proton-conducting membrane. This invention is based on a need to improve the proton conduction ability of fuel cell electrodes that may or may not contain proton conducting ionomeric binder, thereby improving fuel cell performance and durability.